Recently, with the marked progress in a variety of electronic equipment, researches in a rechargeable secondary battery, as a battery that can be used conveniently and economically for prolonged time, are underway. Typical of the known secondary batteries are a lead battery, an alkali storage battery and a lithium secondary battery.
Of these secondary batteries, a lithium secondary battery has advantages in high output and in high energy density. The lithium secondary battery is made up at least of positive and negative electrodes, containing active materials capable of reversibly introducing and removing lithium ions, and a non-aqueous electrolyte.
Nowadays, a compound having an olivinic structure, such as, for example, a compound represented by a general formula LixMyPO4, where x is such that 0<x≦2 and y is such that 0.8≦y≦1.2, with M containing a 3d transition metal, is retained to be promising as a positive electrode active material for a lithium secondary battery.
It has been proposed in Japanese Laying-Open Patent H-9-171827 to use e.g., LiFePO4, among the compounds represented by LixMyPO4, as a positive electrode for a lithium ion battery.
LiFePO4 has a theoretical capacity as high as 170 mAh/g and, in an initial state, contains electro-chemically dopable Li per Fe atom, so that it is a material promising as a positive electrode active material for a lithium ion battery.
Up to now, LiFePO4 was synthesized using a salt of bivalent iron, such as iron acetate Fe(CH3COO)2, as a source of Fe as a starting material for synthesis, and on sintering the starting material at a higher temperature of 800° C. under a reducing atmosphere.
However, it is reported in the above publication that, in the battery prepared using LiFePO4, prepared by the above method for synthesis, as the positive electrode active material, the real capacity only on the order of 60 mAh/g to 70 mAh/g may be realized. Although the real capacity of the order of 120 mAh/g has been reported in Journal of the Electrochemical Society, 144, 1188 (1997), this real capacity cannot be said to be sufficient in consideration that the theoretical capacity is 170 mAh/g.
If LiFePO4 is compared to LiMn2O4, LiFePO4 has a volumetric density and an average voltage of 3.6 g/cm2 and 3.4 V, respectively, whereas LiMnPO4 has a volumetric density and an average voltage of 4.2 g/cm2 and 3.9 V, respectively, with its capacity being 120 mAh/g. So, LiFePO4 is smaller by approximately 10% in both the voltage and the volumetric density than LiMn2O4. So, with the same capacity if 120 mAh/g, LiFePO4 is smaller than LiMn2O4 by not less than 10% in weight energy density and by not less than 20% in volumetric energy density. Thus, for realizing an equivalent or higher level in LiFePO4 with respect to LiMn2PO4, a capacity equal to or higher than 140 mAh/g, is required, however, such a high capacity has not been achieved with LiFePO4.
On the other hand, with LiFePO4, synthesized on sintering at a higher temperature of 800° C., there are occasions where crystallization proceeds excessively to retard lithium diffusion. So, with the non-aqueous electrolyte secondary battery, sufficiently high capacity has not been achieved. Moreover, if the sintering temperature is high, the energy consumption is correspondingly increased, while a higher load is imposed on e.g., a reaction apparatus.